Your normal ultrasound, upper GI endoscopy, colonoscopy, and stool tests are reassuring and make a serious structural bowel disease unlikely. At 24 years of age, considering that you have a healthy diet, the most likely diagnosis is functional constipation or slow-transit constipation. Lactitol (Gut Clear) may take 24–72 hours to become effective, so 2 days without a bowel movement is not unusual. Continue taking it exactly as prescribed, maintain good hydration, regular physical activity, and adequate dietary fibre unless your doctor has advised otherwise. Do not stop or switch back to Cremaffin without discussing it with your gastroenterologist.